The invention relates to a roller drive unit for conveying objects and in particular to a roller drive unit for conveying cargo containers on a roller conveyor track of an aircraft.
Roller drive units of this type are used in particular for transporting items of freight that have been placed on roller-type conveyor tracks. Such items may comprise cargo containers or pallets. A preferred application of the type of roller drive unit disclosed here is in air freight loading systems where the unit is used to transfer containers into the cargo space of an aircraft and stow them for transport. Accordingly, the roller drive unit is installed in the aircraft itself.
When roller drive units of this kind are switched on, drive rollers of the unit are swiveled upward in such a way that they press against the bottom of a cargo container situated above them. This frictional contact enables the torque of the drive rollers to be transmitted to the floor of the cargo container, so that the drive rollers propel the container forward.
The intended field of application of the present invention presents many problems regarding the construction of such roller drive units. In particular, the drive roller must be pressed against the bottom of the container with sufficient force to ensure frictional engagement between the drive roller and the container, so that the torque made available by the drive roller is converted to a propulsive force.
German patent DE 198 07 228 A1 discloses a roller drive unit in which a separate motor is provided to lift the drive roller from a lower, resting position into an upper, operating position and to ensure frictional engagement with the container. This arrangement is relatively elaborate because it requires an additional electric motor.
Another xe2x80x9cprinciplexe2x80x9d widely employed for such roller drive units lies in the fact that the pressing force is xe2x80x9ctapped offxe2x80x9d from the torque of the motor, so that no separate electric motor is needed to lift the drive roller. A roller drive unit of this kind is disclosed, for example, in DE 41 02 424 A1. Here lifting of the drive roller is accomplished by a cam connected to one output of a planetary gear, the other output of which is connected to the drive roller. The drive roller is slowed down by a brake that rotates, to some extent, with it, so that a certain amount of torque acts on the cam. The torque available to propel an object on the roller conveyor track is thus reduced by the following brake.
DE 197 19 161 A1 discloses a roller drive unit wherein a first controllable brake is provided to slow down the lifting cam and a second controllable brake is provided to block the motor, so that the drive roller as a whole can be blocked in order to brake a container situated above it. In this roller drive unit a following brake is likewise provided, which slows down the driver roller so that when it is raised, a portion of the torque provided by the electric motor is diverted to the lifting cam.
The object of the present invention is to provide a simply constructed roller drive unit wherein the pressing force exerted by the drive roller can be reliably adjusted so that there is a corresponding improvement in the required traction properties.
According to the present invention there is provided a roller drive unit for conveying an object comprising a drive motor; a planetary gear with an input connected to the drive motor and with a first gear output and a second gear output; a drive roller, which is connected to the first gear output, to propel the object; a lifting apparatus connected to the second gear output, to lift the drive roller out of a retracted resting position, in which the drive roller does not contact the object, into a raised operating position in which the drive roller can engage the object; a first controllable brake to slow down the drive roller; and a second controllable brake to keep the lifting apparatus in place.
This design of roller drive unit makes it possible on one hand to utilize the entire driving force for lifting the roller drive unit, while on the other hand, when the roller drive unit is in the raised state, the entire driving force is used for propulsion. This is especially important in view of the fact that the roller drive units concerned here are intended to be installed in very confined surroundings and in a situation in which a high degree of reliability is simultaneously demanded.
Preferably, the first and the second brakes are electrically controllable, a generally available bus preferably being used for this purpose.
Preferably also, the first brake is connected to the drive roller by way of a first braking gear, by way of which the torque acting on the brake can be correspondingly reduced. This enables a high torque to be modulated under precise control with a relatively weak brake.
The first brake acts on the drive path between the output of the planetary gear and the drive roller. It can thus be brought into engagement at any desired position in the drive path. In a first preferred embodiment of the invention the brake is installed in the drive roller, which saves considerable space. The braking gear is also accommodated in the drive roller and is preferably designed as a planetary gear, which corresponds to the rotationally symmetric construction of the drive roller.
The second brake is preferably connected to the lifting apparatus by way of a second braking gear. Thus relatively high torques, which appear particularly at this site, can be absorbed with a simple and easily constructed brake.
The lifting apparatus can take the form of a crank, spindle or the like. Preferably, however, it comprises at least one rotatable cam, which lifts the drive roller directly or by way of a pivoted frame in which the drive roller is rotatably mounted. Such a construction is simple and of small size.
The lifting apparatus preferably comprises a device that acts on the drive roller to move it out of its operating position into its resting position, so that it is not only gravity that presses the drive roller downward when the motor is switched off.
In another embodiment of the invention a spring device with damping is provided at the lifting apparatus, which serves to allow a degree of movement of the drive roller even when the brake is blocked, so as to compensate for irregularities on the bottom of the object to be conveyed.
The brakes are preferably constructed as switchable brakes with a blocking position and a release position, which both simplifies the construction of the brakes and reduces wear and tear on them, while the degree of torque that can be supported is simultaneously higher than in the case of friction brakes.
A preferred embodiment of the invention will now be described with reference to the accompanying drawings.